It has been estimated that in most of today's data centers, electronic equipment racks generate between 5 to 12 KiloWatts (KW) of heat. The newest computers (servers) being used in data centers are smaller and more powerful, and allow for a very dense placement of machines inside an equipment rack enclosure. Some estimates are that future heat loads inside equipment racks could go beyond 20 KW. The cooling systems used in these equipment racks are clearly important to the reliability of these data centers.
Efficiency is one of the most important aspects of a rack cooling system. The high cost of air conditioning precludes simply making and distributing more cold air than is needed for cooling. The most efficient system will deliver the right amount of cold air to the computer rack—no more, and no less.
Determining the ideal cooling rate for an equipment rack is difficult because the amount of heat generated can vary greatly. For example, computers sitting idle generate far fewer watts than those being heavily used. So if the entire rack is idle, the cooling demand will be significantly lower than if the entire rack is hard at work. Additionally, the number and type of equipment installed in the rack may change over time. A rack that is only half full of servers today may be completely full next week.
Another related challenge is due to the fact that the internal heat being generated is exhausted from the rear of each of the computers by cooling fans that are internal to each computer. Obviously, the combined airflow produced by these cooling fans must be properly managed. If this airflow is mismanaged, hot air exhausting from the rear of the computers could migrate to the front of the cabinet where it would mix with the cold air supply, and have a detrimental effect on cooling.
Finally, computer manufacturers typically publish “ambient” conditions for which their equipment is designed. The ideal equipment rack cooling system will thus maintain the desired ambient conditions to the internally monitored computers, while using the minimum amount of cooling necessary. In order to do this, the cooling system must not only provide the proper air temperature, it must also insure there are no pressure differentials between the entrance and the exhaust of the internal computers. In other words, the computers must not be allowed to pressurize the rear of the equipment rack, when all their internal fans are running. This could otherwise cause the airflow in Cubic Feet per Minute (CFM) inside the computers to drop, resulting in elevated internal temperatures. Likewise, the computer fans must not be allowed to pull a vacuum in the front of the rack, or again the CFM inside the computers could drop. Alternatively, the cooling system should neither pressurize the front of the rack, nor create a vacuum in the rear of the rack, as either of these conditions would force more cooling air to circulate than is necessary, resulting in reduced efficiency.